This would be the extreme case of the action of all the sympathetic and antipathetic relations between human beings (leaving out of account social relations in their narrowest sense, which are merely the safeguards of communities) which are not included in the law of sexual affinity. As K generally increases with the strength of congenital relationship, A has a greater value when the individuals are of the same nationality than when they belong to different nationalities. The value of _f^t_ is great in this case, and one can investigate its fluctuations, as, for instance, when two domestic animals of different species are in association; at first it usually stands for violent enmity, or fear of each other (and A has a negative value), whilst later on a friendship may come about. When K = 0 in the formula K . _f^t_ A = ---------, α - β then A = 0, which means that between two living beings of origin too remote there may be no trace of sexual attraction. The provisions of the criminal statute-books, however, in reference to sodomy and bestiality show plainly that even in the case of very remote species K has a value greater than nothing. The formula may apply to two individuals not only not of the same species, but even not of the same order. It is a new theory that the union of male and female organisms is no mere matter of chance, but is guided by a definite law; and the actual complexities which I have merely suggested show the need for complete investigation into the mysterious nature of sexual attraction. The experiments of Wilhelm Pfeffer have shown that the male cells of many cryptogams are naturally attracted not merely by the female cells, but also by substances which they have come in contact with under natural conditions, or which have been introduced to them experimentally, in the latter case the substances being sometimes of a kind with which they could not possibly have come in contact, except under the conditions of experiment. Thus the male cells of ferns are attracted not only by the malic acid secreted naturally by the archegonia, but by synthetically prepared malic acid, whilst the male cells of mosses are attracted either by the natural acid of the female cells or by acid prepared from cane sugar. A male cell, which, we know not how, is influenced by the degree of concentration of a solution, moves towards the most concentrated part of the fluid. Pfeffer named such movements “chemotactic” and coined the word “chemotropism” to include these and many other asexual cases of motion stimulated by chemical bodies. There is much to support the view that the attraction exercised by females on males which perceive them at a distance by sense organs is to be regarded as analogous in certain respects with chemotropism.